1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device used for projectors, note book PCs, monitors and the like, and to a drive method therefor.
2. Description of the Related Art
With the progress of the multimedia era, there has been rapid popularization of liquid crystal display devices from small size devices used in projector apparatus, to large size devices used in notebook PCs, monitors and the like. In particular, with the active matrix type liquid crystal display device which is driven by thin film transistors, since this obtains a high resolution, and high picture quality compared to the simple matrix type liquid crystal display device, these have become the main stream of liquid crystal display devices.
FIG. 59 shows an example of an equivalent circuit for one pixel section of a conventional active matrix type liquid crystal display device. As shown in FIG. 59, the pixel of the active matrix type liquid crystal display device comprises a MOS type transistor (Qn) (referred to hereunder as transistor (Qn) 5904 with a gate electrode connected to a scanning line 5901, one of a source electrode and a drain electrode connected to a signal line 5902, and the other of the source electrode and the drain electrode connected to a pixel electrode 5903, a storage capacitor 5906 formed between the pixel electrode 5903 and a storage capacitor electrode 5905, and a liquid crystal 5908 interposed between the pixel electrode 5903 and an opposing electrode Vcom 5907. Presently, with notebook PCs, which constitute a large practical application market for liquid crystal display devices, normally for the transistor (Qn) 5904, an amorphous silicon thin film transistor (referred to hereunder as an a-SiTFT) or a polysilicon thin film transistor (referred to hereunder as a p-SiTFT) is used. Moreover, for liquid crystal material, a twisted nematic liquid crystal (referred to hereunder as a TN liquid crystal) is used. FIG. 60 shows an equivalent circuit for a TN liquid crystal. As shown in FIG. 60, the equivalent circuit for the TN liquid crystal can be represented by a circuit where a liquid crystal capacitance component Cpix, and a resistance Rr and capacitance Cr are connected in parallel. Here the resistance Rr and the capacitance Cr are components for determining the response time constant of the liquid crystal.
The timing chart for a gate scanning voltage Vg, a data signal voltage Vd, and a voltage of the pixel electrode 5903 (referred to hereunder as the pixel voltage) Vpix, for the case where this TN liquid crystal is driven by the pixel circuit construction shown in FIG. 59, is shown in FIG. 61. As shown in FIG. 61, due to the gate scanning voltage Vg in the horizontal scanning period becoming a high level VgH, the transistor (Qn) 5904 comes on, and the data signal voltage Vd input to the signal line is transferred to the pixel electrode 5903 through the transistor (Qn) 5904. The TN liquid crystal normally operates in a mode which passes light when a voltage is not applied, that is a so called normally-white mode. Here for the data signal Vd, a voltage which gives a high light transmittance through the TN liquid crystal is applied over several fields. When the horizontal scanning period is completed and the gate scanning voltage Vg becomes a low level, the transistor (Qn) 5904 goes off, and the data signal transferred to the pixel electrode 5903 is held by the storage capacitor 5906 and the capacitance Cpix of the liquid crystal. At this time, with the pixel voltage Vpix, at the time when the transistor (Qn) 5904 goes off, a voltage shift referred to as feed-through voltage occurs through the capacitance between the gate and source of the transistor (Qn) 5904. In FIG. 61 this is shown by Vf1, Vf2 and Vf3. The amount of this voltage shift Vf1, Vf2 and Vf3 can be made smaller by designing the value for the storage capacitor 5906 to be large. The pixel voltage Vpix is held until the gate scanning voltage Vg again becomes a high level in the subsequent field period and the transistor (Qn) 5904 is selected. The TN liquid crystal switches in accordance with the held pixel voltage Vpix, and as shown by the light transmittance T1, undergoes a transition from the state where the liquid crystal transmitted light is dark to the state where this is bright. At this time, as shown in FIG. 61, in the holding periods, the pixel voltage Vpix fluctuates slightly in the fields by respective amounts ΔV1, ΔV2 and ΔV3. This is in accordance with the liquid crystal response, and is attributable to the change in the capacitance of the liquid crystal. Normally, in order to make this fluctuation as small as possible, the storage capacitor 5906 is designed with a value 2 to 3 three times larger than the pixel capacitance Cpix. As described above, the TN liquid crystal can be driven by the pixel circuit configuration shown in FIG. 59.
However, as indicated by the change in the light transmittance shown in FIG. 61, with the response time of the TN liquid crystal normally large at from 30 to 100 msec, then there is the problem that in the case where an object moving at high speed is displayed, a residual image occurs and a distinct display is thus not possible. Furthermore with the TN liquid crystal, there is the problem that the viewing angle is narrow. Therefore recently, in order to provide high speed and a wide viewing angle, research and development of liquid crystal materials having polarization, and liquid crystal display devices using such liquid crystal materials has been actively performed. An equivalent circuit for a high speed liquid crystal having polarization can be represented as shown in FIG. 62, by a circuit where a series connected resistance Rsp and capacitance Csp, and a high frequency pixel capacitance Cpix which does not change with rotation of polarization, are connected in parallel. The construction of the equivalent circuit is the same as for the equivalent circuit for the TN liquid crystal previously shown in FIG. 60. However, the resistance Rsp and capacitance Csp which determine the liquid crystal response time are different from those of the TN liquid crystal. Therefore in order to distinguish that these are components participating in polarization response, they are shown as a separate figure.
For such a liquid crystal material having polarization, there is for example, a ferroelectric liquid crystal, an antiferroelectric liquid crystal, a thresholdless antiferroelectric liquid crystal, a distorted helix ferroelectric liquid crystal, a twisted ferroelectric liquid crystal, and a monostable ferroelectric liquid crystal. Of these liquid crystal materials, in particular, with a liquid crystal display device using the thresholdless antiferroelectric liquid crystal, not only does this have high speed and wide viewing angle, but as disclosed for example in the Japanese Journal of Applied Physics, Volume 36 p. 720 referred to hereunder as reference 1, by using an active matrix type drive as shown in FIG. 59, then a gradation display is also possible.
FIG. 63 shows a timing chart for the gate scanning voltage Vg, the data signal voltage Vd, and the pixel voltage Vpix, for the case where a thresholdless antiferroelectric liquid crystal is driven by the conventional pixel circuit construction shown in FIG. 59. As shown in FIG. 63, due to the gate scanning voltage Vg in the horizontal scanning period becoming a high level VgH, the transistor (Qn) 5904 comes on, and the data signal voltage Vd input to the signal line is transferred to the pixel electrode 5903 through the transistor (Qn) 5904. The thresholdless antiferroelectric liquid crystal normally operates in a mode which does not pass light when voltage is not applied, that is a so called normally-black mode. When the horizontal scanning period is completed and the gate scanning voltage Vg becomes a low level, the transistor (Qn) 5904 goes off, and the data signal transferred to the pixel electrode 5903 is held by the storage capacitor 5906 and the high frequency pixel capacitance Cpix of the liquid crystal. At this time, with the pixel voltage Vpix, at the time when the transistor (Qn) 5904 goes off, then as with the beforementioned case of driving the TN liquid crystal, a voltage shift through the capacitance between the gate and source of the transistor (Qn) 5904, referred to as feed-through voltage, occurs. Furthermore, after completing the horizontal scanning period, the pixel voltage Vpix fluctuates slightly in the fields by respective amounts ΔV1, ΔV2 and ΔV3 as shown in FIG. 63, due to reallocation of the electrical load held in the high frequency capacitance Cpix and the electrical load held in the capacitance Csp due to polarization. With the drive method disclosed in reference 1, a drive method for gradation control using the pixel voltage Vpix after this voltage fluctuation is disclosed. At this time, in FIG. 63, the light transmittance changes as shown by T1, and the thresholdless antiferroelectric liquid crystal can be driven by means of the pixel circuit configuration shown in FIG. 59.
As an example of a high speed liquid crystal which does not have polarization, a liquid crystal display device which uses an OCB mode liquid crystal is disclosed in IRDC 97, p. L-66. An OCB mode liquid crystal is one which uses the bend orientation of the TN liquid crystal. Compared to the conventional TN liquid crystal; this can switch one or more columns at high speed. Furthermore, by jointly using bi-axial phase difference compensation films, a wide viewing angle display can be obtained.
Recently, research and development into color liquid crystal display devices with a time division driving method which use a high speed crystal such as a ferroelectric liquid crystal, an OCB mode dielectric liquid crystal or the like, has become intense. For example in Japanese Unexamined Patent Publication No. 7-64051, there is disclosed a liquid crystal display device with a time division driving method which uses a ferroelectric liquid crystal. Moreover, in IRDC 97, p. 37, there is disclosed a color liquid crystal display device with a time division driving method which uses an OCB mode liquid crystal. With the liquid crystal display device with a time division driving method, color display is realized by successively changing the light incident on the liquid crystal to red, green and blue in a period of one field. Therefore, a high speed liquid crystal which responds in at least ⅓ of one field period or less is necessary. In the case where the liquid crystal display device with a time division driving method is applied to a direct viewing type liquid crystal display device such as a notebook PC or a monitor, a color filter is not required and hence a cost reduction for the liquid crystal display device can be achieved. Furthermore, in the case where this is applied to a projector apparatus, then a high aperture efficiency similar to that for a three plate type liquid crystal light bulb, can be realized with a liquid crystal display device with a single plate color display. Hence a small size, light weight, low cost and high brightness liquid crystal projector apparatus can be provided.
In the case where a TN liquid crystal, a ferroelectric liquid crystal having polarization, an antiferroelectric liquid crystal, or a high speed liquid TN crystal which responds within one field period, are driven by the above described conventional pixel construction and drive method, the following problems arise.
In the case where, as described above, the TN liquid crystal is driven by the pixel construction shown in FIG. 59, then as shown in FIG. 61, with the pixel voltage Vpix, the voltage fluctuations of ΔV1, ΔV2 and ΔV3 occur due to the change in the liquid crystal capacitance in the holding periods. The amount of these voltage fluctuations changes depending on the amount for operating the liquid crystal molecules. Therefore even in the case where the same data signal is written in, since this depends on the data signal written into the previous field, a problem arises in that the voltage desired to be actually written to the liquid crystal cannot be continually applied over the holding period. As a result, the light transmittance of the liquid crystal which should become the curve shown by T0 in FIG. 61, actually becomes the curve shown by T1 as mentioned before. Hence it is not possible to have an accurate gradation display. Heretofore, in order to reduce the voltage changes ΔV1, ΔV2 and ΔV3, then a method of solving this by designing to increase the storage capacity has been tried. In this case however, there is the problem that the aperture efficiency is reduced.
Furthermore, in the case where a ferroelectric liquid crystal or an antiferroelectric liquid crystal having polarization is driven, then as shown in FIG. 63, with the pixel voltage Vpix, voltage fluctuations shown as ΔV1, ΔV2 and ΔV3 occur due to the polarization switching in the holding periods. These voltage fluctuations, as described before, are due to reallocation of the electrical load held in the high frequency capacitance Cpix and the electrical load held in the capacitance Csp due to polarization. Here Csp has a large value 5 to 100 times that of Cpix. Therefore the voltage changes ΔV1, ΔV2 and ΔV3 become a large value exceeding 1 to 2 volts, so that it is necessary to make the amplitude of the data signal large. As a result, the power consumption of the liquid crystal display device increases. The requirement also arises to make the signal processing circuit, the peripheral drive circuits and the pixel transistors have a high voltage endurance, so that there is the problem of an increase in cost of the liquid crystal display device. Moreover, since the amount of the voltage fluctuation ΔV1, ΔV2 and ΔV3 changes depending on the data signal written in the previous field, then the light transmittance of the liquid crystal which should become the curve shown by T0 in FIG. 62, actually becomes the curve shown by T1 as mentioned before, so that it is not possible to have an accurate gradation display for each field. Consequently, when applied to a liquid crystal display device with a time division driving method, color display with good color reproducibility cannot be performed.
A problem similar to that with the liquid crystal display device using the abovementioned liquid crystal material having polarization also occurs with a liquid crystal display device using an OCB mode liquid crystal.
In Japanese Unexamined Patent Publication No. 7-64051, there is disclosed a liquid crystal display device which uses a single crystal silicon transistor, in order to solve these problems. However with the construction shown in FIG. 18 of Japanese Unexamined Patent Publication No. 7-64051, there is the problem that resetting of the transistor Q2 which operates as a source follower type amplifier is not done. Therefore if a data signal of a voltage lower than the previously written data signal is input, the transistor Q2 remains in the off condition, so that a voltage corresponding to this data signal cannot be output. Furthermore, with the construction shown in FIG. 18 of Japanese Unexamined Patent Publication No. 7-64051, since the transistor Q2 goes off after the data signal is output to the picture element electrode 10, then when after this the polarization current for the ferroelectric liquid crystal flows, a problem similar to the beforementioned problem occurs in that the voltage of the picture element electrode fluctuates.